PATHOLOGY .AND GENETICS OF WHITE PINE BLISTER RUST 435 



drawings and one photograph of what he interpreted to be fine penetration 

 hyphae through the cuticle of primary leaves or the hypocotyl, but never 

 observed a connection between these structures and internal mycelium. 

 Even Patton and Nicholls (1966) reported possible direct penetration. 

 But all of these reports are now believed by this author to be artifacts 

 or faulty interpretation of strain lines or other optical effects in the 

 cuticle or cell wall. 



Infection of very young and succulent stem tissue was reported as a 

 result of artificial inoculations by Van Arsdel (1968) . Susceptibility 

 decreased with shoot age, and cankers appeared most rapidly on stems 

 inoculated at the tender-shoot stage. The mode of penetration of such 

 tissue has not been determined but, since stomata may occur on such 

 shoots (Clinton and McCormick, 1919) , it is likely that infection of such 

 succulent tissue occurs in the same manner as in the needles. 



PRODUCTION OF INFECTION STRUCTURES 



In the pine needle the characteristic feature of stomatal penetra- 

 tion is the formation of a vesicle in the chamber beneath the guard cells. 

 An infection hypha then develops from the vesicle and grows into the meso- 

 phyll tissue, where it branches to form a mycelium that ramifies between 

 and within the mesophyll cells and eventually into the vascular core. 

 Vesicles have very rarely been seen on the needle surface, but seem 

 invariably to result when a germ tube passes between the guard cells of 

 a stoma. No appressorium is formed by the penetrating germ tube. 



Infection structures have also been observed by the author and his 

 co-worker Pritam Singh {unpublished data) when basidiospores were germi- 

 nated on artificial substrates, such as collodion or cellophane membranes 

 floating on water. Two factors important in influencing the number of 

 infection structures produced are fluctuating temperatures and the contact 

 stimulus provided by the membrane. No vesicles were seen when spores were 

 germinated at a constant temperature of 16° or 20°C in 100% relative 

 humidity on water films on glass slides. Under the same temperature condi- 

 tions, basidiospores germinating on collodion membranes produced typical 

 long germ tubes. A few of these (less that 0.5%) developed vesicle-like 

 swellings on the germ tube. These were terminal bulbous swellings that 

 did not develop further or, in fewer cases, an oval swelling that developed 

 a short infection hyphae. On collodion membranes in temperatures fluctu- 

 ating between 4.5°C at night and 21 to 24°C during the day, vesicles were 

 considerably more abundant and seemed more typical in shape and size of 

 those formed in needles. In a representative experiment, counts of over 

 2,000 spores in each of two trials revealed 79 and 90% germination, 10 and 

 5% of germinated spores with vesicles, and 5.6 and 2.4% of germinated 

 spores with vesicles that had developed infection hyphae. 



The formation of infection structures by germinating basidiospores 

 is apparently influenced by several factors. Since vesicles are not 

 formed at all or only very rarely on glass slides, the germ tube apparently 

 needs some sort of stimulus provided by the guard cells in the needle or 

 by artificial membranes. The chemical composition of the membrane and the 

 solution on which it is floating also influence vesicle formation and, 

 especially, development of infection hyphae. Finally, a diurnal tempera- 

 ture fluctuation from about 4 . 5 to 24°C results in more and better-developed 

 vesicles than when spores are incubated at a constant temperature of 16°C. 

 These are all factors that might act on germinating spores on pine needles 



